CHROMOSOMES AND ECTOPLASM 



fore, the condition of the cytoplasm determines the behavior 

 of the chromosomes. To induce cross-fertilization espe- 

 cially between widely separated species, for most eggs at 

 least, one must impair the integrity of the eggs' ectoplasm. 

 Such impairment means a weakened cytoplasm. As with 

 the aberrant behavior of the chromosomes in experimental 

 polyspermy, so with that in cross-fertilization: it follows 

 injury to the egg's ectoplasm. 



A point here must be emphasized. Too frequently bio- 

 logists speak of the incompatibility of chromosomes to 

 account for the elimination of chromosomes in cross-fer- 

 tilization. As a matter of fact, chromatin may be elimi- 

 nated in straight fertilized eggs. I have found in straight 

 fertilized eggs of Echinarachnius that the whole egg-nucleus 

 may fail to take part in the cleavage-mitosis.^ Such eggs 

 are injured.- Here there can be no question of the "incom- 

 patibility" of chromosomes to foreign cytoplasm. Rather, 

 the chromosomes fail to take part in the ensuing mitoses 

 because of the weakened condition of the cytoplasm pre- 

 viously treated. Work on the effect of temperature and 

 of ether on the sperm-nucleus in the echinid egg are ame- 

 nable to the same interpretation. The behavior of monaster 

 eggs is also a case in point: the abnormal behavior of chro- 

 mosomes is clearly due to injury of the superficial cytoplasm 

 brought about by vigorous shaking at a time after insemina- 

 tion when the ectoplasm is very susceptible to experimental 

 treatment. The effect of radiation referred to above on 

 straight fertilized eggs may be recalled. Here again the 

 aberrant behavior of the chromosomes is in consequence 

 of a definite ectoplasmic injury. Finally, Dubois has shown 

 for the egg of Sciara that chromosomes are normally elimi- 



1 Just, 1924. 



- See also J. Gray on Echinus-egg. 



